Roman square capitals, also called capitalis monumentalis, inscriptional capitals, elegant capitals and capitalis quadrata, are an ancient Roman form of writing, and the basis for modern capital letters.

Square capitals were used to write inscriptions, and less often to supplement everyday handwriting. When written in documents this style is known as Latin book hand. For everyday writing the Romans used a current cursive hand known as Latin cursive. Notable examples of square capitals used for inscriptions are found on the Roman Pantheon, Trajan's Column, and the Arch of Titus, all in Rome. Square capitals are characterized by sharp, straight lines, supple curves, thick and thin strokes, angled stressing and incised serifs. These Roman capitals are also called majuscules, as a counterpart to minuscule letters such as Merovingian and Carolingian.

Before the 4th century, square capitals were used to write de luxe copies of the works of authors categorized as "pagan" by Christians, especially those of Virgil; the only three surviving manuscripts using this letter, among them the Vergilius Augusteus, contain works by Virgil. After the 5th century the square capitals fell out of use, except as a display lettering for titles and chapter headings in conjunction with various script hands for body text: for example, uncials.

Edward Catich is noted for the fullest development of the thesis that the inscribed Roman square capitals owed their form, including the serifs, wholly to the use of the flat brush, rather than to the exigencies of the chisel or other stone cutting tools. Although not universally accepted, the brushed-origin thesis had been proposed in the nineteenth century. Catich made a complete study and proposed a convincing ductus by which the forms were created, using a flat brush and then chisel. He promulgated his views in two works, Letters Redrawn from the Trajan Inscription in Rome and The Origin of the Serif: Brush Writing and Roman Letters.

During the early era of the movable typeprinting press, Roman square capitals became the primary inspiration for the capital letters in early serif typefaces; Roman type, especially that developed by those associated with Aldus Manutius, came to produce a number of typefaces still used to the present day.[7] The 1989 digital typeface Trajan from Adobe is a direct, all-capital adaptation of the Roman square capitals on Trajan's column.[8]

The SPQR inscriptional capitals on the Arch of Titus, c. 81 AD, are an example of inscriptional lettering which would have been infilled with bronze. Note the holes for the "tangs" of the cast bronze letters.

^Johnston, Edward (1906). Writing & Illuminating & Lettering. Macmillan. pp. 268–269, 384, 391. The Roman capitals have held the supreme place among letters for readableness and beauty. They are the best forms for the grandest and most important inscriptions.